The present invention relates to the termination of fiber optic cable by a buffer tube coupling coil to couple the fiber with the buffer tube, thereby preventing fiber retraction.
Fiber retraction refers to the movement of fibers into a cable when the cable is subjected to a tensile load. Fiber retraction occurs when fibers are not properly coupled to the cable. As the cable stretches longitudinally, the fibers want to remain in their un-strained state, and if they are not sufficiently coupled to the cable, the ends of the fibers retract into the cable.
Fiber retraction most commonly occurs in central tube fiber optic cable. A central tube cable typically consists of loose fibers within a buffer tube in the center of the cable. Strength members are usually located outside the buffer tube, typically embedded in the outer jacket. A central tube cable can also consist of multiple fiber bundles, ribbons, or even smaller buffer tubes. Many times this central buffer tube is called a core tube, core buffer tube or even an inner jacket.
When a cable is installed between termination points, the cable is subject to tension. One such installation is an aerial installation, where the cable is subject to tension due to the weight of the cable as well as other loads due to wind and ice build-up on the cable. When subjected to these loads, the cable elongates. The strength members in the cable control the amount of elongation. If the components within the cable are properly coupled together, they will all strain in equal amounts. The strength members, jacket, buffer tube(s) and fibers will all strain equally. However, if the components are not coupled together, it is possible that the components can behave differently from each other. It is possible for the cable components to move relative to each other in order to relieve the strain. A common example is a central tube cable with loose fibers in a central buffer tube. The fibers are typically loosely embedded in a filling gel within that buffer tube.
In FIGS. 1A–1C, the interaction between the fiber 10 and the cable 12 is demonstrated. In FIG. 1A, the case is presented where there is no tension on the cable 12. Here, both the cable strain and the fiber strain are equal to zero. Note that the lengths of the cable and fiber are equivalent in this state. Turning to FIG. 1B, the case is presented where the cable is under tension and the fiber 10 and cable 12 are sufficiently coupled together. In this case, the cable has elongated as a result of the tension force by an amount 14. However, because the fiber and cable are sufficiently coupled, the cable strain is equivalent to the fiber strain. Thus, the relative length of the fiber to the cable remains unchanged, and no retraction is observed. Finally, in the case of FIG. 1C, the fiber 10 and cable 12 are not sufficiently coupled together. As a result, when the tension force is applied, the cable elongates by an amount 16, but the fiber does not elongate by the same amount. This lack of adequate coupling leads to the fiber retracting into the cable.
Fiber retraction can lead to significant disruptions in network function. In the case of aerial installations, the ends of the cable segments are connected to a splice closure. Strength members secure the cable to the closure. When tensile load is applied to the cable due to sag, wind, ice, or some other means, the load is designed to be taken by the strength members. If the fibers are not sufficiently coupled to the cable, the fibers are free to move axially within the central cavity of the cable. If the fibers can move freely, this can result in the fibers being pulled out of splice closures as they retract into the cable. This causes problems with the splice closures and severely, if not detrimentally, affects optical performance of the fiber and cable.
Several methods have been developed to prevent fiber retraction. Such methods include the application of glue-type sealant at the ends of the cable to bond the cable components together, leaving extra slack in the splice closures to compensate for the retractions, and the use of coupling coils. However, each of these conventional methods has their own negative implications.
More specifically, one of the conventional approaches previously used to address the problem of fiber retraction is to implement coupling coils which involves forming a portion of the cable into a loop or a plurality of loops. This results in frictional coupling of the internal fibers to the buffer tube to prevent the fiber from moving relative to the other members of the cable. However, because the minimum bend radius of the cable is dictated by the external members of the cable, such as the jacket and strength members, conventional coupling coils can be quite large and bulky. These conventional coupling coils typically exceed well over 1 foot in diameter. Such large coupling coils are undesirable since they are visually unpleasing in applications where cable is installed in residential areas, such as a fiber to the home installation. Further, in many cases, it is not possible to make large coils on telephone poles or lines due to proximity to other cables and/or right of way restrictions. Moreover, conventional coils require a significant length of cable to be utilized to form the large diameter coil, which results in increased installation expense.
Thus, it is desirable to prevent fiber retraction by achieving coupling between the fiber and the cable without the drawbacks associated with conventional approaches.